1. Field of the Invention
The present invention relates to a driving device for use in an image forming apparatus.
2. Description of the Related Art
In response to the current space saving and downsizing demand, there is a tendency to mount components in a high-density packaging manner in image forming apparatuses such as coping machines or printers. However, the electric components in the image forming apparatuses radiate exothermic heat while the mechanical components radiate frictional heat, and it is necessary to cool those components by circulating cooling air. High-density packaging makes it difficult to circulate cooling air and therefore adequately cool the heat radiating components. Moreover, in small image forming apparatus, because heat is confined to a small area, there are greater chances of rise in temperature inside the image forming apparatus. In other words, reliability of image forming apparatuses can drop with the downsizing.
FIG. 13 is a sectional view of a conventional driving device. The driving device includes a shaft 201 that supports a gear 202 rotatably. The gear 202 is fixed to a bearing boss member 203, and a contact surface 202a, which contacts with the shaft 201, is formed inside the gear 202. The gear 202 includes two gear members 202b and 202c having respectively different diameters. Frictional heat is generated between the shaft 201 and the contact surface 202a of the gear 202 and between the gear 202 and the bearing boss member 203, and temperatures of the shaft 201 and the gear 202 rise.
Linear expansion of the shaft 201 and the gear 202 can occur due to such rise in the temperature, and relative positions of the shaft 201 and the gear 202 can deviate from the original positions. This can lead to acceleration of wear of the shaft and the gear, lowing of the efficiency of force coupling, and lowering of the overall reliability of the image forming apparatus.
Japanese Patent Application Laid-open No. H8-152836 discloses a gear that includes a helical vane member therein to intake, discharge, and cool an air by rotation. Japanese Patent Application Laid-open No. H7-332351 discloses a supporting structure for supporting a rotor, whose shaft is made into cylindrical shape, to cool a frictional heat generated from rotation of a gear. Japanese Patent Application Laid-open No. H9-257048 discloses a cooling structure for cooling a driving device that generates an air flow toward a bearing by rotating a gear with a vane and cools a heating area.
One approach to lower the friction between parts could be to use a ball bearing; however, use of a ball bearing increase the costs.
The technology disclosed in Japanese Patent Application Laid-open No. H8-152836 relates to cooling of a mating area of a gear, but the technology is unable to cool the entire gear or to cool a sliding surface between the gear and a shaft. On the other hand, the technology disclosed in Japanese Patent Application Laid-open No. H7-332351 is limited to a gear with a hollow shaft, and therefore, cannot be applied to a bearing having no hollow portion. Finally, the technology disclosed in Japanese Patent Application Laid-open No. H9-257048 requires that a vane to be added to a gear, which leads to increase in the costs.